On physical and social-cost-based CO2 equivalents for transient albedo-induced forcing

Analyzing land-based climate mitigation measures requires the joint examination of the effects of multiple forcing agents. Non-CO2 forcing agents are often converted into “physical” CO2 equivalents, i.e., expressed in terms of how much CO2 would be required to inflict the same climate forcing as one unit of the agent. Similar conversions can also be based on equal social cost. Social-cost-based CO2 equivalents are ideal for economic applications, as they retain the correct social cost ratio between forcing agents. We present two kinds of social-cost-based CO2 equivalents for albedo-induced (or any other transient) forcing. The emission equivalent expresses transient forcing as equivalent CO2 emissions (tCO2yr−1). The stock equivalent expresses it as an equivalent reduction in the terrestrial carbon stock (tCO2). We compare physical and social-cost-based CO2 equivalents, showing that social-cost-based the emission equivalent is a generalization of its physical counterpart, whereas social-cost-based stock equivalent is not. Nevertheless, the two stock equivalents are structurally similar and of roughly the same magnitude. We discuss how the equivalents can be applied in economic analysis, provide numerical multiplier values for converting transient forcing into emission and stock equivalents, and assess the extent to which physical CO2 equivalents distort the social valuation of transient forcing.